This invention relates to method and apparatus for combining the output of multiple lasers and, more particularly, to coherent aperture filling of coupled and uncoupled laser arrays. Here, aperture filling describes the process of transforming an array of closely spaced and parallel input beams into a single output beam which extends across the entire array and which maintains a uniform electric field amplitude and phase transversely. In the far field such an aperture-filled beam propagates with a dominant single-lobed transverse intensity distribution (weak side lobes given just by diffraction from the array exit aperture) as opposed to a strongly multi-loaded grating pattern generated by the separated beams of the laser array without aperture filling. Generally, such side lobes are to be avoided, since they reduce the energy in the central lobe. In addition, energy in the side lobes can be detrimental in certain applications.
A laser array, ideal in the sense of generating a single-lobed far-field pattern, would have all lasers in phase, exhibit uniform amplitude of the laser modes, and feature a 100% "fill factor". The "fill factor" is that fraction along the transverse length of the laser array where light is emitted. In actual coupled arrays, all of the lasers are in phase; but the amplitude of the laser modes are Gaussian (TEM00) or some other nonuniform distribution; furthermore, the fill factor must always be less than 100%, due to operational constraints, and typically is in the 25-75% range. For a coupled array having a 40% fill factor, for example, the far field distribution will have a central lobe energy of approximately 40%; the rest of the energy is distributed in the side lobes.
One technique, well-known in the art, for enhancing, in principle, the fill factor of a laser array is the precise placement of a matched lens system in front of each laser in the array such that individual output beams are collimated. This approach is, however, cumbersome, in particular for a large number of lasers in a small array, and, most importantly, cannot achieve the desirable 100% fill factor, due to boundary constraints at the lens edges and the nonuniform nature of the individual beams.
Another approach for achieving a high power laser beam is to superimpose coherently the output of plural lasers. In U.S. Ser. No. 662,609 issued Mar. 10, 1987 as patent No. 4,649,351, of which two of the present applicants are co-inventors, there is disclosed and claimed apparatus for the coherent addition of uncoupled arrays of lasers. As disclosed in that patent application, external feedback is required to lock together the phase of the individual lasers. The output according to the teachings of that patent application is a single laser beam having an 80-90% efficiency and a transverse dimension identical to that of a single laser of the array.
For the present aperture filling technique, to be described below, both uncoupled laser arrays with an external optical feedback cavity and phase-coupled laser arrays are applicable. There is no restriction on the spatial distribution of the lasers in the array: one-, two-, or even three-dimensional configurations can be accommodated, the latter three-dimensional case with the restriction that sufficient depth-of-field of the transform optics be provided; also, the individual laser elements need not be located in a regular array, nor have identical output apertures. The present aperture filling technique is also applicable for side lobe suppression of an angularly steerable beam emanating from a phased laser array.
Coupled laser arrays are presently being developed as one-dimensional arrays including plural closely spaced, coupled lasers in which the light from all of the lasers is in phase. Such phase-locked arrays are known for both gallium arsenide (evanescently coupled and Y-guide) diode lasers and CO.sub.2 (ridged waveguide) gas lasers. For these lasers systems the amount of available optical output power from a single laser is restricted to an upper limit by physical constraints which cannot be circumvented. Thus the only alternative towards higher output levels is the combination of several gain or laser sections in multi-stage series or parallel array configurations.
It is therefore a principal object of this invention to provide apparatus for coherent aperture filling of laser arrays which highly suppresses side lobes in the far-field distribution.
Yet another object of the invention is such apparatus which results in over 90% of the energy being contained in the central lobe.
Still another object of this invention is such apparatus which can be readily implemented with existing fabrication techniques and which is applicable for both phase-locked and unlocked laser arrays.